The present invention relates to a process for making microcrystalline cellulose (MCC). More specifically, the present invention relates to a process where microcrystalline cellulose is made by reactive extrusion.
Microcrystalline cellulose is obtained by hydrolysis of the amorphous portion of cellulose until a level-off degree of polymerization (LODP) product is obtained. MCC is comprised of highly crystalline regions of cellulose and usually has a LODP of 200-300.
Microcrystalline cellulose is useful for a number of different applications. Pieces of microcrystalline cellulose easily bond together without the use of an adhesive. Furthermore, MCC can be mixed with other substances so as to hold an additive while bonding to itself. It can be made into pharmaceutical-grade tablets, such as vitamins, pain relievers, and other medicines. It also may be used as a substitute for starch where starch is used as a smoothener, such as in suntan lotion.
The hydrolysis of cellulose to obtain MCC can be accomplished using mineral acid, enzymes or microorganisms. Although enzymatic methods are desirable because glucose, a useful by-product, is created, these methods are more expensive and create MCC products having a lower crystallinity. Thus, acid hydrolysis is the conventional method of choice for manufacturing MCC.
In conventional methods, MCC is formed by reacting cellulose with acid in a batch-type reaction vessel. Specifically, a large amount of acid solution is placed in a vat and then cellulose is added. Next, the acid hydrolyzes the cellulose, and MCC is formed. Such processes require significantly greater amounts of acid solution than cellulose. When this batch-type process is complete, a large amount of acid solution remains.
Acids that can be used in this process include hydrochloric acid, sulfuric acid, and phosphoric acid. At higher temperatures, sulfuric acid and phosphoric acid can peptize and surface modify (esterify) MCC. However, such MCC is difficult to purify and does not have the same visual and functional properties as underutilized cellulose produced with hydrochloric acid. One example of using hydrochloric acid involves a method using 2.5N hydrochloric acid, which is able to specifically cleave , 1-4 glucosidic linkages. More product can be created with such hydrochloric acid than with sulfuric acid at the same conditions. However, if used at high concentrations, hydrochloric acid is a strong corrosive agent.
MCC obtained by acid hydrolysis using conventional methods has a particle size of about 200 microns, although particle size can vary somewhat depending on the starting material used. The MCC can undergo mechanical grinding until the particles are of a desirable size.
Mechanical grinding can be accomplished by mixing the MCC with a water solution in either a blender or in a hammer mill. The MCC particle shape obtained after grinding depends mostly on the nature of the raw starting material, especially upon whether the cellulose is in its native or regenerated state.
Conventional methods do not contemplate the advantage of using pressure and high shear forces created by an extruder to aid in performing this acid hydrolysis reaction. Methods presently available also have extremely harmful impacts on the environment because they require large amounts of acid and thus create significant quantities of acid waste.
A process for producing microcrystalline cellulose is needed that has a shorter reaction time than previous processes. Furthermore, a process is needed that is a continuous process rather than a batch-type process. Still further, a process using a limited amount of acid is needed. In addition, a process that is able to produce small particles of microcrystalline cellulose without the need for mechanical grinding also would be advantageous.